The present invention relates to a method and an apparatus for determining the flow characteristics of fluid flowing in a well, particularly the fluid produced in an oil well.
It has long been known that if an obstacle of suitable shape is placed across a single-phase flow, vortices are emitted alternately on each side of the obstacle with a frequency f proportional to the flow velocity. This is the phenomenon referred to as Karman vortex streets. By detecting the passage of the vortices, it is thus possible to determine the fluid flowrate.
The characteristics of flowmeters operating according to this principle are enumerated in the article by R. S. Medlock: "The Vortex Flowmeter--Its Development and Characteristics" which appeared in the Australian Journal of Instrumentation and Control, April 1976, pages 31-32. Among the advantages may be mentioned, notably, the extensive measurement range (100:1), the total absence of moving parts, and the calibration simplicity.
These advantages are particularly valuable for flowrate measurement in a hydrocarbon well using a sonde suspended from a transmission cable. A vortex emission sonde adapted to these very specific measurement conditions forms the subject of copending U.S. patent application Ser. No. 327,973, filed on Dec. 7, 1981. The sonde described therein uses a differential pressure transducer to detect the passage of the vortices.
The sonde described in that application comprises, as a transducer sensitive to the passage of the vortices, a differential pressure transducer.
However, the fact that the fluid produced by an oil well is frequently a gas-liquid mixture raises a problem. It has been noted that in the case of a gas-liquid mixture, the proportionality relationship between the liquid flow rate Q and the frequency f is modified in that the ratio f/Q increases slightly with the proportion of bubbles Y. At least an approximate knowledge of the bubble rate is hence necessary for determining the flow rate with satisfactory accuracy.
It should be noted that the bubble rate constitutes valuable information in itself. The measurement of this proportion at different depths makes it possible to detect the bubble point, i.e. the level at which the reduction in hydrostatic pressure allows the gas, until then dissolved in the liquid, to begin to form a distinct phase.
As disclosed, e.g. in U.S. Pat. No. 4,169,376, the amplitude of a signal produced by a pressure transducer associated with a vortex shedding obstacle is, in the case of a monophasic fluid, proportional to the fluid density and to the square of the flow rate. By processing the signal to substantially suppress the dependency of the amplitude on the flow rate, a reduced amplitude Ar proportional to the fluid density will be obtained. But in itself, such a dependency on the density rather implies a low sensitivity of the amplitude to the presence of bubbles. In the presence of bubbles the mean density of the fluid shows a decrease, but the relative decrease is of the same order as the bubble rate, and in fact is slightly lower since the gas density is not fully negligible in the conditions of a well.